Magnetic Pulse Welding and Spot Welding with Improved Coil Eﬃciency—Application for Dissimilar Welding of Automotive Metal Alloys

Journal of Manufacturing and Materials Processing

Article Magnetic Pulse Welding and Spot Welding with Improved Coil Eﬃciency—Application for Dissimilar Welding of Automotive Metal Alloys

Chady Khalil, Surendar Marya and Guillaume Racineux *

Research Institute in Civil and Mechanical Engineering (GeM, UMR 6183 CNRS), Ecole Centrale de Nantes, 1 rue de la Noë, F-44321 Nantes, France; [email protected] (C.K.); [email protected] (S.M.) * Correspondence: [email protected]

Received: 4 June 2020; Accepted: 6 July 2020; Published: 8 July 2020

Abstract: Lightweight structures in the automotive and transportation industry are increasingly researched. Multiple materials with tailored properties are integrated into structures via a large spectrum of joining techniques. Welding is a viable solution in mass scale production in an automotive sector still dominated by steels, although hybrid structures involving other materials like aluminum are becoming increasingly important. The welding of dissimilar metals is difficult if not impossible, due to their differential thermo mechanical properties along with the formation of intermetallic compounds, particularly when fusion welding is envisioned. Solid-state welding, as with magnetic pulse welding, is of particular interest due to its short processing cycles. However, electromagnetic pulse welding is constrained by the selection of processing parameters, particularly the coil design and its life cycle. This paper investigates two inductor designs, a linear (I) and O shape, for the joining of sheet metals involving aluminum and steels. The O shape inductor is found to be more efficient both with magnetic pulse (MPW) and magnetic pulse spot welding (MPSW) and offers a better life cycle. Both simulation and experimental mechanical tests are presented to support the effect of inductor design on the process performance.

Keywords: magnetic pulse welding; spot welds; linear coils; shear lap test; automotive alloys; numerical analysis; LS-DYNA

1. Introduction The emissions targets set by various climate summits, which started in the 1970s, as well as the increase in oil prices since then, has made the automotive OEMs focus their efforts on optimizing vehicle emissions and keeping fuel consumption at a minimum to meet both the environmental and end customers’ economic concerns. These efforts were translated by extensively optimizing the combustion engines, introducing the hybrid concept, and introducing, during the 1990s, the first commercial electric vehicles. The common helping factor for all the versions proposed was reducing the weight of the structural and non-structural components by using a combination of materials. The metal percentage of the materials used in the production of a typical passenger vehicle, even by the year 2025, is expected to maintain a big part of the share: 60% steel (combination of low-carbon, mild-, and high-strength steels), 18% for aluminum, and 5% magnesium [1]. Consequently, the use of these materials implies the need to deal with the challenges related to dissimilar metal joining by overcoming the constraints stemming from unmatching properties (mechanical, thermal, and chemical) and by ensuring manufacturing conditions (limit the damage on joining partners, reliable and cost-efficient processes, and recyclability).

J. Manuf. Mater. Process. 2020, 4, 69; doi:10.3390/jmmp4030069 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 69 2 of 20

The traditional joining technologies which are well established in the industry are the mechanical and thermal techniques. The former includes processes such as screwing, riveting, punch riveting and clinching. The main disadvantages of these processes are the additional steps required in the production cycle, stress concentrations at the points of fastening, and weight increase inherent to the fasteners themselves [2,3]. For fusion welding technologies that implicitly involve a heat source with or without a filler addition, the main concern is microstructural changes, residual stresses and defects during the solidification of the molten zone between the adjoining components. For instance, weld fusion zones are not always exempt from porosity, solidification cracks and oriented structures with grain orientations that induce unpredictable variability in the welded components [4,5]. When a filler is additionally used to mitigate weld metal chemistry, the weight increase is inherent, as in mechanical joints. In automotive applications of fusion welding, resistance spot welding has been in practice for a very long time and, for the last two decades, remote laser spot welding is increasingly preferred due to the processing speed [6,7]. Further lightweight automotive body structures imply hybrid materials as diverse as aluminum and steels and need fast joining technologies to keep pace with the large-scale automotive sector [8]. The fusion welding of such dissimilar materials is more complex due to the formation of intermetallic compounds in the weld zone and the presence of differential thermal effects stemming from different physical properties, like the thermal expansion coefficient and melting points. This leads to the unacceptable weakening of the welded components. In the case of aluminum to steel thermal joining application, two important limitations need to be mentioned: the formation of brittle, aluminum, rich-intermetallic compounds (Fe2Al5, FeAl3) and the negligible solid solubility of Fe in Al [9]. For a viable dissimilar joint, one possible way of doing this is to reduce, as much as possible, the size of the intermetallic phases. For this to happen, amongst other alternatives to minimize the formation of brittle compounds, some prospective studies involve laser roll bonding through brazing [10] and mixing, as in friction spot welding [11], but with limited success in large-scale applications. To overcome the mechanical and thermal limitations, the studies of high-velocity impact processes which create a direct joining between metals at solid-state and at microscopic level presented an opportunity. These studies started first in the 1950s with the explosive welding (EXW) [12], and continued with magnetic pulse welding (MPW), laser impact welding (LIW) [13], and vaporizing foil actuator welding (VFAW) [14]. The cold and rapid natures of these processes—some microseconds—limit the risk of HAZ and intermetallic compounds’ formation, respectively [15]. The principle of the high-velocity impact processes is based on accelerating one metal, called a flyer, at very high velocities, towards another fixed metal, called parent, where the local progressive collision creates the bond between the two materials. The difference lies in the accelerator types: in EXW, for example, the detonators are used, while in MPW, electromagnetic driving forces are used [2–5]. For safety and ergonomic reasons of massive industrial applications, the MPW process presents itself as a more appropriate candidate, as it uses the electromagnetic fields as a source. The driving force generation is based on the Laplace force principle: the presence of a current-carrying conductive metal in a time-varying magnetic field generates Laplace forces on this metal. Therefore, and to achieve large amounts of Laplace forces, the MPW processes use:

A high pulse generator capable of generating an intense, time-varying current that will be the • source of the time-varying magnetic field; A coil capable of producing the magnetic field due to the discharged current delivered by • the generator; A conductive metal in the vicinity of the coil in which the magnetic field penetrates and induces • currents, leading to the generation of large amounts of Laplace forces, causing its acceleration.

MPW for a tubular geometry was the most used and developed in the MPW field [16,17]. Starting in the year 2000, more focus was given to the sheet metal applications where the operational positioning J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 21 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 3 of 21 J. Manuf.MPW Mater. for Process. a tubular2020 ,geometry4, 69 was the most used and developed in the MPW field [16,17]. Starting3 of 20 in theMPW year for 2000, a tubular more geometry focus was was given the most to the used sheet and developedmetal applications in the MPW where field the[16,17]. operational Starting positioninginis presentedthe year is2000, inpresented Figure more1a,b infocus Figure and was Figure 1a,b given and2A. Figure Manogaranto the 2A.sheet Manogaran et metal al. [ 18applications ]et developed al. [18] developed where an additional the an operational additional process processpositioningconcept concept for metalis presented for sheets: metal in Magnetic sheets: Figure Magnetic Pulse1a,b and Spot PulseFigure Welding Spot 2A. (MPSW). WeldingManogaran In(MPSW). this et al. process, [18] In this developed a priorprocess, local an astamping prioradditional local is stampingprocesscreated onconcept is the created intended for onmetal the spot-welding sheets:intended Magnetic spot-welding location Pulse in the Spotlocation flyer Welding metal, in the which (MPSW).flyer metal, is called In whichthis the process, humpis called (Figurea priorthe hump1 localc,d); (Figurestampingthis hump 1c,d); is will created this guarantee hump on the will theintended guarantee required spot-welding stando the requiredff distance, location standoff and in the hencedistance, flyer the metal, flyerand whichhence could isthe be called directlyflyer the could placedhump be directly(Figureon theparent 1c,d);placed this metal on humpthe without parent will anyguaranteemetal concern without th fore required any the overlappingconcern standoff for thedistance, distance overlapping betweenand hence distance the the two fl betweenyer materials could the be to twodirectlycomply materials withplaced the to on comply industrial the parent with ease themetal for industrial automated without ease any applications. fo concernr automated for This the applications. hump overlapping will then This distance be hump accelerated betweenwill then by the thebe acceleratedtwomagnetic materials pulse by tothe to comply magnetic create with a spot pulse the weld industrialto create after collision a ease spot fo weldr (Figure automated after2B). collision applications. (Figure This2B). hump will then be acceleratedAfterAfter this thisby thebrief brief magnetic description description pulse of of to the thecreate MPW/MPSW, MPW a spot/MPSW, weld the theafter process’s collision operational (Figure 2B). parameters parameters can can be be concluded:concluded:After this thethe electricalbriefelectrical description coil’s coil’s geometry, ofgeometry, the MPW/MPSW, the configuration’sthe confi guration’sthe process’s geometrical geometrical operational parameters parameters parameters and the dischargeand can the be concluded: the electrical coil’s geometry, the configuration’s geometrical parameters and the dischargeenergy of energy the generator. of the generator. These operational These operational parameters parameters condition condition the collision the mechanism, collision mechanism, where the discharge energy of the generator. These operational parameters condition the collision mechanism, wheretwo impact the two physical impact parameters physical parameters are the impact are velocitythe impact and velocity the impact and angle, the impact which angle, are mandatory which are to where the two impact physical parameters are the impact velocity and the impact angle, which are mandatoryensure two to clean ensure surfaces two clean ready surfaces for welding ready and for the welding formation and the of the formation jet [2–6, 19of ].the jet [2–6,19]. mandatory to ensure two clean surfaces ready for welding and the formation of the jet [2–6,19]. ItIt is is worth mentioningmentioning that that the the impact impact would would invariably invariably generate generate heat heat by theby transformationthe transformation from It is worth mentioning that the impact would invariably generate heat by the transformation frommechanical mechanical to thermal to thermal energy, energy, thereby thereby limiting limit theing highest the impacthighest velocityimpact thatvelocity would that not would engender not from mechanical to thermal energy, thereby limiting the highest impact velocity that would not engendermelting at melting the welded at the interface. welded Forinterface. dissimilar For dissim joints, asilar mentioned joints, as mentioned earlier, melting earlier, has melting to be curtailed has to engender melting at the welded interface. For dissimilar joints, as mentioned earlier, melting has to bein ordercurtailed to eliminate in order the to possibleeliminate formation the possible of intermetallic formation compounds.of intermetallic It is compounds. worth noting It that, is worth due to be curtailed in order to eliminate the possible formation of intermetallic compounds. It is worth notingshort impactthat, due times, to short heat impact generation times, remains heat genera nearlytion adiabatic remains andnearly localized adiabatic in and the interfaciallocalized in zone. the noting that, due to short impact times, heat generation remains nearly adiabatic and localized in the interfacialIn the case zone. of Al In to the Cu impactcase of joining,Al to Cu Marya impact et joining, al. [20] haveMarya reported et al [20] localized have reported melting inlocalized tube to interfacial zone. In the case of Al to Cu impact joining, Marya et al [20] have reported localized meltingtube welding, in tube which to tube has welding, been substantiated which has been by others substantiated [15,19]. Briefly, by others the velocity[15,19]. Briefly, range for the successful velocity melting in tube to tube welding, which has been substantiated by others [15,19]. Briefly, the velocity rangewelding for hassuccessful a limited welding window has and a limited consequently window an and equal consequently choice of parameters an equal likechoice stando of parametersff distance, range for successful welding has a limited window and consequently an equal choice of parameters likedischarge standoff energy distance, and configurationdischarge energy of sheets and configuration/parts. of sheets/parts. like standoff distance, discharge energy and configuration of sheets/parts.

FigureFigure 1. 1. ((aa,b,b)) Magnetic Magnetic Pulse Pulse Welding Welding (MPW); (MPW); ( (cc,,dd)) Magnetic Magnetic Pulse Pulse Spot Spot Welding Welding (MPSW).(MPSW) Figure 1. (a,b) Magnetic Pulse Welding (MPW); (c,d) Magnetic Pulse Spot Welding (MPSW)

Figure 2. (A) Al/Cu MPW [21]; (B) MPSW [18].

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 4 of 21

J. Manuf. Mater. Process. 2020, 4, 69 Figure 2. (A) Al/Cu MPW [21]; (B) MPSW [18] 4 of 20

Objectives of Study Objectives of Study The MPW/MPSW for sheet metal applications has been successfully applied and achieved for similarThe MPWand dissimilar/MPSW for metals sheet metalwelding applications [1,4,7–10,22]: has aluminum been successfully to alumin appliedum, aluminum and achieved to steel, for similaraluminum and dissimilarto magnesium. metals In the welding literature, [1,4, 7a– lot10 ,of22 al]:loys aluminum were tested, to aluminum, and the applications, aluminum in to general, steel, aluminumconcerned to thin magnesium. sheet metal In the thicknesses, literature, omitting a lot of alloys the real were thicker tested, sheets and the that applications, can be applied in general, in real concernedapplications thin for sheet the automotive metal thicknesses, industry. omitting On the othe the realr hand, thicker the most sheets used that electrical can be appliedcoil during in realthese applicationsstudies were for linear the automotive with rectangular industry. cross-section On the other coils, hand, which the mostpresent used a good electrical efficiency coil during to simplicity these studiesratio for were aluminum linear with 1xxx rectangular and thin sheet cross-section applications coils, [1,11]. which present a good efficiency to simplicity ratio forHowever, aluminum in 1xxxautomotive and thin applications, sheet applications the 5000- [1,11]. and 6000- series aluminum alloys’ use is dominantHowever, (body in automotive panels, seat applications, structures, the 5000-reinforc andement 6000- seriesmembers, aluminum etc.). alloys’On the use steel is dominant side, the (bodylow-carbon panels, seatdrawing structures, steels reinforcementare used for various members, comp etc.).onents On theand, steel during side, the the last low-carbon three decades, drawing the steelsdevelopments are used for of variousthe advanced components high-strength and, during steels the (AHSS) last three led to decades, the increase the developments in the use of the of thedual advancedphase steels high-strength (DP) [23]. steels (AHSS) led to the increase in the use of the dual phase steels (DP) [23]. InIn this this context, context, the the current current study study aimed aimed to to develop develop an an electrical electrical coil coil geometry geometry allowing allowing a bettera better effiefficiencyciency and and wider wider application application scope scope of of the the MPW MPW/MPSW/MPSW for for the the automotive automotive metal metal alloys. alloys. The The new new geometrygeometry coils, coils, as as will will be be seen seen in in the the results, results, led led to to a widea wide range range of of successful successful welds welds for for di differentfferent combinationscombinations of similarof similar and and dissimilar dissimilar alloys. alloys. Several Several combinations combinations were alsowere tested also tested under quasi-static,under quasi- dynamicstatic, dynamic and fatigue andloads fatigue to loads see the to welds see the strengths welds strengths level. level. First,First, the the equipment equipment used, used, including including the two the coils two (linear coils and (linear the newly and designed), the newly the designed), experimental the setupexperimental (Figure3) andsetup an (Figure LS-Dyna 3) and numerical an LS-Dyna model numerical which will model be used which in the will analysis be used of in the the improved analysis of effitheciency improved of the newefficiency coil design, of the willnew be coil presented. design, wi Afterll be that, presented. we will presentAfter that, the we comparison will present of the the twocomparison coils and proceedof the two to thecoils experimental and proceed results.to the experimental results.

Parent Metal Parent Metal Insulator Insulator Flyer Metal Flyer Metal Flyer Metal

Coil Coil

a. b.

Figure 3. Experimental setup: (a) MPW; (b) MPSW. Figure 3. Experimental setup: (a) MPW; (b) MPSW 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials The aluminum and steel alloys chemical compositions used during this study are presented in Tables1The–3. Theiraluminum mechanical and steel properties alloys chemical are given composit in Tablesions4 and used5. during this study are presented in Table 1, Tables 2 and 3. Their mechanical properties are given in Tables 4 and 5. Table 1. Aluminum alloys, chemical compositions (%at.). Table 1. Aluminum alloys, chemical compositions (%at.) Mn Mg Other Si max Fe max Cu max Cr max Zn max Ti max max max max 1050 0.25 0.40 0.05 0.05 0.05 - 0.07 0.05 0.03 5182 0.20 0.35 0.15 0.50 4.0–5.00 0.10 0.25 0.10 0.15 5754 0.40 0.40Si max 0.10Fe max Cu max 0.50Mn max 2.6–3.6Mg max 0.30 Cr max 0.20Zn max Ti max 0.15 Other max - 6013 0.781050 0.280.25 0.40 0.97 0.05 0.400.05 1.02 0.05 0.06 - 0.07 0.10 0.05 0.05 0.03 0.15 6016 0.5–1.55182 0.20 0.50 0.35 0.20 0.15 0.200.50 0.25–0.70 4.0–5.00 0.10 0.10 0.25 0.20 0.10 0.15 0.15 0.15 5754 0.40 0.40 0.10 0.50 2.6–3.6 0.30 0.20 0.15 -

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Table 2. DC04 steel chemical compositions (%at.).

C max Mn max Si max P max S max Al max DC04 0.08 0.40 0.10 0.025 0.025 0.020

Table 3. DP steels chemical compositions (%at.).

C Mn Si P S Ti + Nb V Cr Mo B N Ni Nb Al max max max max max max max max max max max max max max DP450 0.10 0.16 0.4 0.04 0.015 0.015–0.08 0.05 0.01 0.8 0.3 0.005 0.008 -- DP1000 0.139 1.50 0.21 0.009 0.002 0.046 - 0.01 0.02 - 0.0002 0.003 0.03 0.015

Table 4. Steel’s mechanical properties.

YS (MPa) UTS (MPa) A % ISO 20 80 × DC04 (e 1.47 mm) 160–200 280–340 37 ≤ DP450 290–340 460–560 27 DP1000 787 1059 8.5

Table 5. Aluminum alloys’ mechanical properties.

YS (MPa) UTS (MPa) A % ISO 20 80 × 1050-H14 85 105 4 5754-H111 (e 1.5 mm) 90–130 200–240 21 ≤ 5182 (e 1.5 mm) 120–160 260–310 23 ≤ 6013-T4 174 310 26 6016-T4 110–150 220–270 23

2.2. Equipment and Experimental Procedure

2.2.1. Pulse Generator The pulse generator used is a 50 kJ, developed at ECN, with the following characteristics:

CGen = 408 µF, LGen = 0.1 µH; RGen = 3 mΩ; Vmax = 15 kV; Imax = 500 kA; fshort = 25 kHz.

The highest limit for the discharge energy is hence ﬁxed at 16 kJ, so that the discharge current does not exceed 80% of the maximum allowable current for the generator:

I = 0.8 Imax = 400 kA operationmax × 2.2.2. Coils The two coils used in this study are a linear rectangular cross-section coil—dimensions are presented in Figure4—and an O-shaped rectangular cross-section coil—dimensions are presented in Figure5. The active areas of the two coils are the same: 20 8 mm. × 2.2.3. Discharge Energy and Discharge Current Relation The discharge energy E depends on the voltage that is charging the capacitors and it can be expressed by 1 E = C V2 (1) discharge 2 Gen 0 where CGen is the generator capacitance and V0 is the charging voltage. The resulting discharge current in the coil, which is highly damped sinusoidal due to the RLC equivalence of the circuit, is J. Manuf. Mater. Process. 2020, 4, 69 6 of 20

t I(t) = I0e− τ sin(ωt) (2)

where r C I = V Gen (3) 0 0 L 2L τ = (4) R 1 f ω = = (5) √LCGen 2π with L and R as the equivalent inductance and resistance of the system, respectively

L = LGen + LCoil (6) J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 21 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEWR = RGen + RCoil 6 of(7) 21

Figure 4. Linear rectangular cross-section coil FigureFigure 4.4. LinearLinear rectangularrectangular cross-sectioncross-section coil.coil

FigureFigure 5. 5. O-shapeO-shape coil coil dimensions dimensions. Figure 5. O-shape coil dimensions 2.2.3.2.2.4. Discharge Experimental Energy Procedure and Discharge and Design Current Relation 2.2.3. Discharge Energy and Discharge Current Relation TheIn the discharge case of theenergy MPW (Figure depends1a,b), on the the conﬁguration voltage that involvesis charging the usethe ofcapacitors insulators and to createit can thebe expressedneededThe airgap dischargeby between energy the two depends metals (Figure on the3). voltage The insulators that is char are madeging the of PE capacitors and PVC and and it they can are be expressed by 1 = 1 (1) =2 (1) 2 where is the generator capacitance and is the charging voltage. whereThe resulting is the dischargegenerator currentcapacitance in the and coil, whic is theh ischarging highly dampedvoltage. sinusoidal due to the RLC equivalenceThe resulting of the circuit, discharge is current in the coil, which is highly damped sinusoidal due to the RLC equivalence of the circuit, is () = sin() (2) () = sin() (2) Where Where

= (3) = (3) 2 = 2 (4) = (4)

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 21 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 7 of 21 1 = = (5) 1 2 = = (5) 2 with and as the equivalent inductance and resistance of the system, respectively with and as the equivalent inductance and resistance of the system, respectively = + (6)

= + (6) = + (7)

= + (7) 2.2.4. Experimental Procedure and Design 2.2.4. Experimental Procedure and Design J. Manuf.In Mater.the case Process. of 2020the ,MPW4, 69 (Figure 1a,b), the configuration involves the use of insulators to create7 of 20 the neededIn the case airgap of thebetween MPW the(Figure two metals1a,b), the (Figure configuration 3). The insulators involves theare usemade of ofinsulators PE and PVCto create and thethey needed are designed airgap tobetween have the the same two thickness metals (Figure of the 3).required The insulators standoff distanceare made between of PE and metals. PVC They and designed to have the same thickness of the required standoff distance between metals. They are fixed theyare fixed are designed on the flyer to have metal the using same adhesive thickness tapes. of th Ine requiredFigures 6 standoff and 7, the distance positioning between of the metals. flyer metalThey on the flyer metal using adhesive tapes. In Figures6 and7, the positioning of the flyer metal regarding areregarding fixed on the the coil flyer is representedmetal using adhesivefor both linear tapes. and In Figures O-shaped 6 and coils, 7, the respectively. positioning of the flyer metal the coil is represented for both linear and O-shaped coils, respectively. regarding the coil is represented for both linear and O-shaped coils, respectively.

Figure 6. Magnetic Pulse Welding configuration with linear coil. Figure 6. Magnetic Pulse Welding conﬁguration with linear coil. Figure 6. Magnetic Pulse Welding configuration with linear coil.

Figure 7. Magnetic Pulse Welding configuration with O-shape geometry coil. Figure 7. Magnetic Pulse Welding configuration with O-shape geometry coil. After the cleaningFigure 7. step,Magnetic the flyerPulse metalWelding is configurat positionedion facing with O-shape the coil geometry where a coil. Kapton insulation sheetAfter with athe 0.1 cleaning mm thickness step, the is fl usedyer metal to separate is positioned it from thefacing coil. th Thee coil positioning where a Kapton is controlled insulation by asheet laserAfter with so that thea 0.1 thecleaning mm part thickness ofstep, the the flyer is flusedyer metal metalto toseparate be is deformedpositioned it from is thefacing centered coil. th Thee regardingcoil positioning where thea Kapton coil’sis controlled active insulationarea. by a Thesheet parent with metala 0.1 mm is then thickness positioned is used on theto separate insulators, it from over itthe a massivecoil. The steel positioning die, and is finally controlled the whole by a system is clamped using a special system designed to avoid any displacement during the welding process. The standoff distance is controlled using a feeler gauge before every test. In case of MPSW (Figure1c,d and Figure3b), the first step of the process is to create the hump in the flyer metal. The general geometry of the hump chosen is a rectangular one. This geometry was chosen based on the previous study done by Manogaran et al. [19], during which different geometries were tested, and it was proven that the rectangular one is the more efficient. Figure8 presents this general geometry with different dimensions. The hump is stamped in the flyer metal using a hydraulic press die. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 21 laser so that the part of the flyer metal to be deformed is centered regarding the coil’s active area. The parent metal is then positioned on the insulators, over it a massive steel die, and finally the whole system is clamped using a special system designed to avoid any displacement during the welding process. The standoff distance is controlled using a feeler gauge before every test. In case of MPSW (Figure 1c,d and Figure 3b), the first step of the process is to create the hump in the flyer metal. The general geometry of the hump chosen is a rectangular one. This geometry was chosen based on the previous study done by Manogaran et al. [19], during which different geometries were tested, and it was proven that the rectangular one is the more efficient. Figure 8 presents this J. Manuf. Mater. Process. 2020, 4, 69 8 of 20 general geometry with different dimensions. The hump is stamped in the flyer metal using a hydraulic press die.

FigureFigure 8. 8. GeneralGeneral geometry geometry of of the the hump hump for for MPSW. MPSW. After cleaning the metal surfaces from oil using acetone solution, the flyer metal is then positioned After cleaning the metal surfaces from oil using acetone solution, the flyer metal is then with a laser so that it is facing and centered on the coil’s active area. The parent metal is then positioned positioned with a laser so that it is facing and centered on the coil’s active area. The parent metal is above the flyer metal with the massive die on, and finally the system is clamped like the MPW case. then positioned above the flyer metal with the massive die on, and finally the system is clamped like the2.2.5. MPW Welds case. Strength Evaluation

2.2.5.For Welds the weldingStrength mechanical Evaluation strength evaluation, the welded specimens were tested as schematically represented in Figure9 in lap-shear conditions: For the welding mechanical strength evaluation, the welded specimens were tested as schematicallyQuasi-static represented at 10 2 mm in/ sFigu (withre an9 in Instron lap-shear 5584 conditions: mechanical tensile machine, Norwood, MA, USA); • − Dynamic at 614 mm /s (with an MTS 819 hydraulic high-speed tensile machine, Eden Prairie, •• Quasi-static at 10 mm/s (with an Instron 5584 mechanical tensile machine, Norwood, MA, MN, USA); USA); • Fatigue under unidirectional conditions (R = 0, frequency of 20 Hz, Fmax = 0.6 Fmaxquasi static • Dynamic at 614 mm/s (with an MTS 819 hydraulic high-speed tensile machine, Eden× Prairie, −MN, J. Manuf.USA);with Mater. an Process. MTS Electropulse2020, 4, x FOR PEER E10000 REVIEW machine, Eden Prairie, MN, USA). 9 of 21 • Fatigue under unidirectional conditions (=0, frequency of 20 Hz, =0.6× with an MTS Electropulse E10000 machine,100 Edenmm Prairie,30 MN, mm USA).

Welding Flyer Parent zone 50 mm F F a.

100 mm 30 mm

Welding Flyer Parent zone 12 mm 50 mm F F

12 mm b.

Figure 9. (a) quasi static and fatigue lap shear tests specimen, (b) dynamic lap-shear test specimen. Figure 9. (a) quasi static and fatigue lap shear tests specimen, (b) dynamic lap-shear test specimen

2.2.6. Numerical Simulation The model used is a coupled mechanical/thermal/electromagnetic using LS-DYNA 3D FEM- BEM code (Finite Element Method – Boundary Element Method). The simulations during this study were performed using the High-Performance Computing (HPC) resources of the Centrale Nantes Supercomputing Center on the cluster Liger by the High- Performance Computing Institute (ICI).

2.3. Materials Properties and Boundary Conditions The coils material used in this study are the OFHC copper and the ASTM A36 steel. The flyer metals used in the numerical models are the aluminum alloys 5754 and 5182. The parent metal is the DC04 deep drawing law carbon steel. A Johnson–Cook model was used for the mechanical solver and the parameters for all materials are taken from the literature [24–27] and they are presented in Table 6. The thermal and electrical parameters of different materials are presented in Tables 7 and 8 and they are also taken from the literature [24–27]. The initial room temperature was set at 293 K.

Table 6. Johnson–Cook model parameters

Coil Alloy A (MPa) B (MPa) n C m Linear Coil OFHC 90 292 0.31 0.025 1.09 OFHC 90 292 0.31 0.025 1.09 O-Shape Coil ASTM A36 286.1 500.1 0.2282 0.022 0.917 5754 67.456 471.242 0.424 0.003 2.519 Flyers 5182 106.737 569.120 0.485 −0.001 3.261 Parent DC04 162 598 0.6 2.623 0.009

Table 7. Thermal properties.

Coil Alloy Thermal Conductivity Specific Heat Capacity (W/mk) (J/kgK) Linear Coil OFHC 386 383 OFHC 386 383 O-Shape Coil ASTM A36 50 450 Flyers 5754 130 897

J. Manuf. Mater. Process. 2020, 4, 69 9 of 20

2.2.6. Numerical Simulation The model used is a coupled mechanical/thermal/electromagnetic using LS-DYNA 3D FEM-BEM code (Finite Element Method – Boundary Element Method). The simulations during this study were performed using the High-Performance Computing (HPC) resources of the Centrale Nantes Supercomputing Center on the cluster Liger by the High-Performance Computing Institute (ICI).

2.3. Materials Properties and Boundary Conditions The coils material used in this study are the OFHC copper and the ASTM A36 steel. The ﬂyer metals used in the numerical models are the aluminum alloys 5754 and 5182. The parent metal is the DC04 deep drawing law carbon steel. A Johnson–Cook model was used for the mechanical solver and the parameters for all materials are taken from the literature [24–27] and they are presented in Table6. The thermal and electrical parameters of diﬀerent materials are presented in Tables7 and8 and they are also taken from the literature [24–27]. The initial room temperature TRT was set at 293 K.

Table 6. Johnson–Cook model parameters.

Coil Alloy A (MPa) B (MPa) n C m Linear Coil OFHC 90 292 0.31 0.025 1.09 O-Shape OFHC 90 292 0.31 0.025 1.09 Coil ASTM A36 286.1 500.1 0.2282 0.022 0.917 5754 67.456 471.242 0.424 0.003 2.519 Flyers 5182 106.737 569.120 0.485 0.001 3.261 − Parent DC04 162 598 0.6 2.623 0.009

Table 7. Thermal properties.

Thermal Conductivity Speciﬁc Heat Capacity Coil Alloy (W/mk) (J/kgK) Linear Coil OFHC 386 383 OFHC 386 383 O-Shape Coil ASTM A36 50 450 5754 130 897 Flyers 5182 123 902 Parent DC04 52 470

Table 8. Electrical Properties—International Annealed Copper Standard.

%IACS Coil Alloy S/m (International Annealed Copper Standard) Linear Coil OFHC 100 5.8001 107 × OFHC 100 5.8001 107 O-Shape Coil × ASTM A36 12 6.9600 106 × 5754 33 1.9140 107 Flyers × 5182 28 1.6240 107 × Parent DC04 13 7.5400 106 ×

The variation in the electrical conductivity, with temperature for the copper and aluminum, is based on the Meadon model, which is a simpliﬁed Burgess model [28], and it gives the conductivity as a function of temperature and density at solid phase [29] ! V C3 ρ = C1 + C2T fc (8) V0 J. Manuf. Mater. Process. 2020, 4, 69 10 of 20

where ρ is the electrical resistivity, T is the temperature, V is the speciﬁc volume, V0 is the reference speciﬁc volume (zero pressure, solid phase) and

! !2γ 1 V V − fC = (9) V0 V0 where γ is the Gruneisen value given by ! 1 V γ = γ0 γ0 1 (10) − − 2 − V0 with γ0 the reference Gruneisen value. In Table9, the set of parameters for aluminum and copper from Burgess paper are given [20,21].

Table 9. Meadon-Burges parameters.

Parameter Cu Al 3 V0 (cm /g) 0.112 0.370 γ0 2 2.13 5 5 C1 4.12 10 5.35 10 − × − − × − C2 0.113 0.223 C3 1.145 1.210

For the steel case, a simple linear model was used

ρ = ρ [1 + α (T TRT)] (11) 0 0 − where ρ is the electrical resistivity at the actual temperature T, ρ0 is the electrical resistivity at the 3 1 reference temperature TRT and α = 6 10 K− is the temperature coeﬃcient of resistivity for the 0 × − taken TRT reference temperature. The source currents in the coil are due to the imposition of boundary conditions, where it is possible to have an imposed current or an imposed voltage [29]. In our case, the voltage V(t) is imposed through an equation depending on the charging voltage V0, the resistance RGen, the inductance LGen and capacity CGen of the generator, as well as the mesh resistance and inductance (Figure 10). Dirichlet andJ. Manuf. Newman Mater. boundary Process. 2020 conditions, 4, x FOR PEER are REVIEW applied, and no further constraint is applied on the BEM [ 1130 of]. 21

Figure 10. Problem with (R, L, C) imposed voltage on the coil [28]. Figure 10. Problem with (R, L, C) imposed voltage on the coil [28] The inductor nodes’ movement was constrained in all directions (translation and rotation), since in the MPWThe applications, inductor nodes’ the coils movement are fixed was in constrained such a way in as al tol preventdirections their (translation movement. and It rotation), is the same since in the MPW applications, the coils are fixed in such a way as to prevent their movement. It is the same for the parent metal nodes. The flyer sheet nodes’ movement in MPW overlapping case (Figure 11) was blocked outside the distance that represents the distance between the insulators. In the case of the MPSW (Figure 12), and since the hump is the only part that will be accelerated from the flyer, all the other nodes were also constrained.

Figure 11. Model in the MPW overlapping configuration with O shape coil

Figure 12. Model cross section view in the MPSW configuration with O shape coil

2.4. Mesh and Time Step The electromagnetic solver uses eight nodes’ brick elements only, and hence the mesh was controlled to not have any other type of elements in the model. Since we are in a 3D full-coupled

simulation, and to avoid very long simulation times, the length of one element edge in the

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 21

Figure 10. Problem with (R, L, C) imposed voltage on the coil [28] Figure 10. Problem with (R, L, C) imposed voltage on the coil [28] J. Manuf. Mater. Process. 2020, 4, 69 11 of 20 The inductor nodes’ movement was constrained in all directions (translation and rotation), since in theThe MPW inductor applications, nodes’ movement the coils are was fixed constrained in such a inway all asdirections to prevent (translation their movement. and rotation), It is the since same forinfor the thethe MPW parent parent applications, metal metal nodes. nodes. the The Thecoils flyerfl areyer fi sheetsheetxed in nodes’nodes’ such a movement movementway as to prevent inin MPWMPW their overlappingoverlapping movement. casecase It is (Figure (Figurethe same 1111) ) wasforwas the blocked blocked parent outside outside metal thenodes. the distance distance The fl thatyer representssheet that represents nodes’ the movement distance the distance between in MPW between the overlapping insulators. the insulators. case In the (Figure caseIn the of 11) case the MPSWwasof the blocked MPSW (Figure outside (Figure 12), and the 12), since distance and the since hump the that ishump represents the only is th parte the only thatdistance part will that bebe tweenwill accelerated be the accelerated insulators. from the from In flyer, the the allcase fl theyer, otherofall the the nodes MPSW other were nodes (Figure also were 12), constrained. also and constrained.since the hump is the only part that will be accelerated from the flyer, all the other nodes were also constrained.

Figure 11. Model in the MPW overlapping configuration with O shape coil FigureFigure 11.11. Model in the MPW overlapping configuration conﬁguration with with O O shape shape coil coil.

Figure 12. Model cross section view in the MPSW conﬁguration with O shape coil. FigureFigure 12.12. Model Model cross cross section section view view in in the the MPSW MPSW configuration configuration with with O Oshape shape coil coil 2.4. Mesh and Time Step 2.4. Mesh andand TimeTime StepStep The electromagnetic solver uses eight nodes’ brick elements only, and hence the mesh was The electromagneticelectromagnetic solversolver usesuses eighteight nodes’ nodes’ brick brick elements elements only, only, and and hence hence the the mesh mesh was was controlled to not have any other type of elements in the model. Since we are in a 3D full-coupled controlled toto notnot havehave anyany otherother type type of of elements elements in in the the model. model. Since Since we we are are in in a a3D 3D full-coupled full-coupled simulation, and to avoid very long simulation times, the length of one element edge le in the simulation, andand toto avoidavoid veryvery longlong simulatisimulationon times, times, the the length length of of one one element element edge edge in inthe the non-active zones was equal to the skin depth δ, and in the active zones it was smaller to have accurate numerical results: δ l = (12) e 3 The time-step ∆t for the electromagnetic solver is computed as the minimal elemental diﬀusion time step over the elements [30] µ0σ ∆t = l 2 (13) 2 e where µ0 is the magnetic permeability of the material and σ its electrical conductivity.

3. Results and Discussion

3.1. Numerical Comparison of Linear and O-Shape Coils Efficiencies The presence of a current-carrying conductive metal in a time-varying magnetic field generates Laplace forces on this metal. The Laplace force acting on an elementary volume dV is expressed in (14). In the MPW case, the flyer metal is the current-carrying conductive where the mentioned current is induced due to the time-varying magnetic field generated by the discharged current in the coil. Consequently, and in accordance with the following Equation (14), J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 12 of 21

non-active zones was equal to the skin depth , and in the active zones it was smaller to have accurate numerical results: = (12) 3 The time-step Δ for the electromagnetic solver is computed as the minimal elemental diffusion time step over the elements [30]

Δ = ² (13) 2

where is the magnetic permeability of the material and its electrical conductivity.

3. Results and Discussion

3.1. Numerical Comparison of Linear and O-Shape Coils Efficiencies The presence of a current-carrying conductive metal in a time-varying magnetic field generates J. Manuf.Laplace Mater. forces Process. on2020 this, 4 metal., 69 The Laplace force acting on an elementary volume is expressed12 of 20 in (14). In the MPW case, the flyer metal is the current-carrying conductive where the mentioned current is induced due to the time-varying magnetic field generated by the discharged current in the coil. Consequently, and in accordance with thed→F following= →J →B dVEquation (14), (14) × = × (14) The forces generated are directly proportional to the induced current density in the flyer metal and theThe magnetic forces inductiongenerated generatedare directly by proportional the discharge to current, the induced which current are also density related, in since the theflyer latter metal isand the sourcethe magnetic of the former.induction generated by the discharge current, which are also related, since the latter is theGoing source back of to the the former. Laplace force equation, the O-shape coil design presented in Figure 13 is based on the ideaGoing of back improving to the Laplace efficiency force by equation, having higher the O-sh dischargeape coil currents design presented and induced in Figure currents 13 is at based the sameon the levels idea of of energies improving based efficiency on: by having higher discharge currents and induced currents at the same levels of energies based on: Reducing the inductance of the coil to increase the peak currents (3); • • PuttingReducing more the charge inductance carriers of the on thecoil flyerto increase metal inthe movement peak currents to increase (3); the induced currents • • magnitudePutting more and charge increasing, carriers at the on samethe flyer time, metal the current in movement density to in increase the area wherethe induced the material currents shouldmagnitude be accelerated. and increasing, at the same time, the current density in the area where the material should be accelerated.

Figure 13. O-Shape rectangular cross-section. Figure 13. O-Shape rectangular cross-section The measured inductance of the linear coil presented a value equal to 81% of the generator’s The measured inductance of the linear coil presented a value equal to 81% of the generator’s inductance LGen, giving a system total inductance of inductance , giving a system total inductance of L = 1.81L (15) 1 = 1.81Gen (15)

The measured inductance of the O-shape coil presented a value equal to 49% of the generator’s inductance LGen, giving a system total inductance of

L2 = 1.49LGen (16)

Comparing the inductances, the system with the O-shape had a 10% higher peak current and frequency. L2 = 0.82L1 (17) I ω 02 = 2 = 1.1 (18) I01 ω1 To compare the efficiency of the O-shape vs. the linear coil, the developed numerical model was used to compare the flyer velocities in the MPW configuration case for the thick aluminum 5754-H111 (e f = 1.2 mm). Since the discharge currents are higher in the O-shape coil case, the mechanical strength of the coil should be higher, and hence two approaches were considered. In the first, the material of the coil is kept as OFHC copper, but the thickness was increased to 4 mm (i.e., 1.4 times the thickness in the linear coil case). The second approach involves the use of a 4-mm thickness coil made of steel ASTM J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 13 of 21

The measured inductance of the O-shape coil presented a value equal to 49% of the generator’s

inductance , giving a system total inductance of

= 1.49 (16) Comparing the inductances, the system with the O-shape had a 10% higher peak current and frequency.

= 0.82 (17)

= =1.1 (18) To compare the efficiency of the O-shape vs. the linear coil, the developed numerical model was used to compare the ﬂyer velocities in the MPW conﬁguration case for the thick aluminum 5754-H111

( =1.2 mm). Since the discharge currents are higher in the O-shape coil case, the mechanical J. Manuf.strength Mater. Process. of the2020 coil, 4 ,should 69 be higher, and hence two approaches were considered. In the fi13rst, of the 20 material of the coil is kept as OFHC copper, but the thickness was increased to 4 mm (i.e., 1.4 times the thickness in the linear coil case). The second approach involves the use of a 4-mm thickness coil A36, whichmade hasof steel better ASTM mechanical A36, which properties, has better and mechanic seeing whetheral properties, the loss and in electrical seeing whether conductivity the loss will in have aelectrical high influence conductivity on its will global have effi a ciency.high influence on its global efficiency. FigureFigure 14 presents 14 presents a comparison a comparison between between impact impact velocities velocities for the for cases the of cases use ofof an use OFHC of an copper OFHC O-shapecopper and O-shape a linear OFHCand a linear copper OFHC coil. copper The velocities coil. The atvelocitiesE = 10 kJat in the = 10 O-shape kJ in the coil O-shape case are coil much case higherare than much these higher at E than= 16 thesekJ inat the = linear 16 kJ case. in the The linear velocity case. The inthe velocity O-shape in the case O-shape is, on case average, is, on 1.75 timesaverage, the velocity1.75 times in the the velocity linear coil in the shape. linear coil shape. WhenWhen using using a steel a steel O-shape O-shape coil, coil, the the velocities velocities are are 1.55 1.55 times times the the velocities velocities in in the the linear linear coil coil case.case. The velocities’The velocities’ curves curves at di ffaterent different stando standoffff distances distan inces both in both cases cases are are presented presented in in Figure Figure 15 15.. The dataThe presenteddata presented in Figures in Figures 14 and 14 and15 show 15 show clearly clearly that that the the O-shaped O-shaped coil coil is is far far better better than than the the linear coil and, even when steel is used for the coil, it still has higher efficiency than the linear coil and, even when steel is used for the coil, it still has higher efficiency than the linear coil. linear coil.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 14 of 21 FigureFigure 14. Impact 14. Impact velocity velocity comparison comparison between between the the OFHC OFHC O-shape O-shape and an lineard linear coils coils (e f(= =1.21.2 mm). mm)

FigureFigure 15. Impact15. Impact velocities velocities comparison comparison between between the the steel steel O-shape O-shape coil coil and and linear linear copper copper coilcoil (e f = 1.2( =1.2 mm). mm)

3.2. Experimental Validation The first step in the experimental analysis was to explore the weldability between various metal sheets with various mechanical properties and thicknesses, using both coils in the same range of

discharge energies (4 kJ ≤ ≤16 kJ). The configuration used is the MPSW and the hump dimensions (Figure 8) are: =8 mm; =12 mm; ’ =10 mm, ’ =18 mm, and ℎ =1.6 mm. Table 10 summarizes the results for different combinations. The limit of the linear coil can be observed when flyer metal thickness exceeds 1.2 mm, while the O-shape coil succeeded the welds up to 2 mm.

Table 10. Weldability of different metals alloys using copper linear coil (L) and O shape coil (: not welded / : welded / -: not tested)

Parent 5754 5182 6016 DP450 DP1000 DC04 Flyer L O L O L O L O L O L O 5754 (0.5 mm)             5182 (1 mm)   - - - -   - -   5182 (1.2 mm)   - - - -   - -   5182 (1.4 mm)   - - - -   - -   5182 (2 mm)   - - - -   - -   6013 (1.4 mm) ------    6016 (1 mm)   ------ 

3.3. Mechanical Testing for the Welding Joints Now that the weldability of a variety of combinations is proven using the O-shape coil for thick metal sheets, the strength of welds will be tested. As the focus is the dissimilar welding for automotive applications, five combinations are tested in quasi-static, dynamic, and fatigue lap-shear configuration:

J. Manuf. Mater. Process. 2020, 4, 69 14 of 20

3.2. Experimental Validation The first step in the experimental analysis was to explore the weldability between various metal sheets with various mechanical properties and thicknesses, using both coils in the same range of discharge energies (4 kJ E 16 kJ). The configuration used is the MPSW and the hump ≤ discharge ≤ = = = = = dimensions (Figure8) are: lh 8 mm; Lh 12 mm; lh0 10 mm, Lh0 18 mm, and hh 1.6 mm. Table 10 summarizes the results for different combinations. The limit of the linear coil can be observed when flyer metal thickness exceeds 1.2 mm, while the O-shape coil succeeded the welds up to 2 mm.

Table 10. Weldability of diﬀerent metals alloys using copper linear coil (L) and O shape coil (: not welded/X: welded/-: not tested).

Parent 5754 5182 6016 DP450 DP1000 DC04 Flyer LOLOLOLOLOLO 5754 (0.5 mm) XXXXXXXXXXXX 5182 (1 mm) XX ---- XX -- XX 5182 (1.2 mm) XX ---- XX -- XX 5182 (1.4 mm) X ---- X -- X 5182 (2 mm) X ---- X -- X 6013 (1.4 mm) ------ X X 6016 (1 mm) XX ------XX

3.3. Mechanical Testing for the Welding Joints Now that the weldability of a variety of combinations is proven using the O-shape coil for thick metal sheets, the strength of welds will be tested. As the focus is the dissimilar welding for automotive applications, ﬁve combinations are tested in quasi-static, dynamic, and fatigue lap-shear conﬁguration:

Aluminum 5182 (e = 1.2 mm) with steel DC04 (ep = 0.8 mm) in MPSW configuration; • f Aluminum 6016 (e = 1 mm) with steel DC04 (ep = 0.8 mm) in MPSW configuration; • f Aluminum 5182 (e = 1.2 mm) with steel DP450 (ep = 1.17 mm) in MPSW configuration; • f Aluminum 6013-T4 (e = 1.4 mm) with steel DP1000 (ep = 1 mm) in MPSW configuration; • f Aluminum 6013-T4 (e = 1.4 mm) with steel DP1000 (ep = 1 mm) in MPW configuration. • f

For the five combinations, the same steel O-shape coil and the same discharge energy (Edischarge = 16 kJ) are used. For the MPSW configurations, the same hump dimensions are used: lh = 12 mm; Lh = 20 mm; = = = lh0 40 mm, Lh0 55 mm, and hh 1.6 mm. For the MPW configuration, the distance between insulators is D =18 mm and the standoff distance is h = 1.4 mm. In each case, three specimens are tested, and the results are presented in Table 11 for quasi-static lap-shear tests, in Table 12 for dynamic lap-shear tests and in Table 13 for fatigue tests. For 6016/XES dynamic tests, the failure occurred at the aluminum fixation holes at 9000 N without any failure in the welding for all the tested specimens. The 5182/DC04 weld shows, in the quasi-static lap shear tests, maximum loads between 6250 and 6750 N, with a displacement between 1.62 and 1.7 mm. The failure was in the weld and a deformation of the steel in the welding area was observed. The maximum dynamic loads that the weld attained were between 7900 and 8300 N, with a displacement between 1.38 and 1.53 mm, and the failure occurred in the welding also. Finally, the number of cycles reached during the fatigue tests was between 25,000 and 28,000 cycles, where a tearing is observed in both aluminum and steel plates during the test (Figure 16). 5182/DP450 weld shows in the quasi-static lap shear tests maximum loads between 9200 and 10,000 N and the displacements are between 1.32 and 1.88 mm. The failure occurred in the welds in all J. Manuf. Mater. Process. 2020, 4, 69 15 of 20 the 3 tests. The maximum dynamic loads were between 6100 and 8500 N and the displacements were between 1.36 and 2.9 mm. The fatigue tests showed that the number of cycles is between 21,000 cycles and 24,000 cycles.

Table 11. Quasi-static lap-shear tests

Fmax—Quasi-Static Lap-Shear Test Materials Welding Average Average Specimen 1 Specimen 2 Specimen 3 (Flyer/Target) Conﬁguration Fmax ∆Lmax Fmax ∆Lmax Fmax ∆Lmax Fmax ∆Lmax (N) (mm) (N) (mm) (N) (mm) (N) (mm) 5182/DC04 MPSW 6500 1.63 6250 1.56 6500 1.7 6750 1.62 5182/DP450 MPSW 9400 1.64 10,000 1.88 9000 1.32 9200 1.73 6016/DC04 MPSW 8433 2.15 8500 2.15 8700 2.3 8100 2 6013/DP1000 MPSW 7800 0.98 8400 1.1 7600 1.06 7400 0.77 6013/DP1000 MPW 8767 0.98 9500 1.1 9200 0.95 7600 0.9

Table 12. Dynamic lap-shear tests.

Fmax—Dynamic Lap-Shear Test (0.614 m/s) Materials Welding Average Average Specimen 1 Specimen 2 Specimen 3 (Flyer/Target) Conﬁguration Fmax ∆Lmax Fmax ∆Lmax Fmax ∆Lmax Fmax ∆Lmax (N) (mm) (N) (mm) (N) (mm) (N) (mm) 5182/DC04 MPSW 8067 1.46 8300 1.48 7900 1.53 8000 1.38 5182/DP450 MPSW 7033 1.88 8500 2.9 6500 1.37 6100 1.36 6016/DC04 MPSW >9000 >9000 >9000 >9000 6013/DP1000 MPSW 7667 3.85 9300 2.75 7400 3.6 6300 5.2 6013/DP1000 MPW 14,233 1.7 14,600 2.32 14,400 1.3 13,700 0.78

Table 13. Fatigue tests results (R = 0; f = 20 Hz).

Materials Welding Fmax Number of Cycles (Flyer/Target) Conﬁguration (kN) Average Specimen 1 Specimen 2 Specimen 3 5182/DC04 MPSW 4 26,333 26,000 28,000 25,000 5182/DP450 MPSW 5 22,333 21,000 24,000 22,000 6016/DC04 MPSW 4.4 43,000 44,000 42,000 43,000 6013/DP1000 MPSW 4.4 31,000 32,000 30,000 31,000 6013/DP1000 MPW 5 64,333 61,000 69,000 63,000

6016/DC04 weld shows in the quasi-static lap shear tests maximum loads between 8100 N and 8700 N and the displacements are between 2 mm and 2.3 mm. The fatigue tests showed number of cycles between 42,000 and 44,000 and tearing was observed in both aluminum and steel sheets (Figure 17). Concerning the dynamic test in this case and for all tested specimens the failure occurred at the aluminum ﬁxation holes at 9000 N without any failure in the welding. 6013/DP1000 welds were tested in both MPW and MPSW conﬁgurations.

In MPW case: the welds showed quasi-static maximum loads between 7600 and 9500 N with • displacements between 0.9 and 1.1 mm. The dynamic loads are between 13,700 and 14,600 N and the corresponding displacements are between 0.78 and 2.32 mm. The fatigue tests showed the number of cycles oscillating between 61,000 and 69,000; In the MPSW case: the welds showed quasi-static maximum loads in this case were between 7400 • and 8400 N and the displacements were between 0.77 and 1.1mm. The dynamic loads attained were between 6300 and 9300 N and the displacements were between 2.75 and 5.2 mm. The fatigue tests showed a number of cycles between 30,000 and 32,000. J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 16 of 21

The 5182/DC04 weld shows, in the quasi-static lap shear tests, maximum loads between 6250 and 6750 N, with a displacement between 1.62 and 1.7 mm. The failure was in the weld and a deformation of the steel in the welding area was observed. The maximum dynamic loads that the weld attained were between 7900 and 8300 N, with a displacement between 1.38 and 1.53 mm, and the failure occurred in the welding also. Finally, the number of cycles reached during the fatigue tests was between 25,000 and 28,000 cycles, where a tearing is observed in both aluminum and steel plates during the test (Figure 16). 5182/DP450 weld shows in the quasi-static lap shear tests maximum loads between 9200 and 10,000 N and the displacements are between 1.32 and 1.88 mm. The failure occurred in the welds in all the 3 tests. The maximum dynamic loads were between 6100 and 8500 N and the displacements were between 1.36 and 2.9 mm. The fatigue tests showed that the number of cycles is between 21,000 cycles and 24,000 cycles. 6016/DC04 weld shows in the quasi-static lap shear tests maximum loads between 8100 N and 8700 N and the displacements are between 2 mm and 2.3 mm. The fatigue tests showed number of cycles between 42,000 and 44,000 and tearing was observed in both aluminum and steel sheets (Figure 17). Concerning the dynamic test in this case and for all tested specimens the failure occurred at the aluminum ﬁxation holes at 9000 N without any failure in the welding. 6013/DP1000 welds were tested in both MPW and MPSW configurations. • In MPW case: the welds showed quasi-static maximum loads between 7600 and 9500 N with displacements between 0.9 and 1.1 mm. The dynamic loads are between 13,700 and 14,600 N and the corresponding displacements are between 0.78 and 2.32 mm. The fatigue tests showed the number of cycles oscillating between 61,000 and 69,000; • In the MPSW case: the welds showed quasi-static maximum loads in this case were between 7400 and 8400 N and the displacements were between 0.77 and 1.1mm. The dynamic loads attained J. Manuf.were Mater. between Process. 63002020 ,and4, 69 9300 N and the displacements were between 2.75 and 5.2 mm. The fatigue16 of 20 tests showed a number of cycles between 30,000 and 32,000.

J. Manuf. Mater. Process.FigureFigure 2020, 4 ,16.16. x FOR Aluminum PEER REVIEW 5182 and steel DC04DC04 tearingtearing duringduring fatiguefatigue tests.tests 17 of 21

Figure 17.17. Aluminum 6016 and steel DC04 tearingtearing during fatigue tests.tests

3.4. Inductor Life Cycle 3.4. Inductor Life Cycle For heavy duty applications prevalent in automotive industry, the inductor must be capable of sustaining repeatedrepeated shocks shocks during during impulse impulse welding. welding. As mentioned As mentioned earlier inearlier Section in3, theSection acceleration 3, the wasacceleration imparted was to theimparted flyer metal to the to flyer produce metal impact to produce velocities, impact particularly velocities, in particularly the case of hard in the materials case of hardand/or materials thicker sections,and/or thicker which sect requiresions, eitherwhich increasing requires either the discharge increasing current the discharge and/or reducing current theand/or coil reducingcross-section. the coil Subsequently, cross-section. the Subsequently, coil will then itselfthe coil experiment will then higher itself thermo-mechanicalexperiment higher thermo- stresses mechanicalbecause of thestresses action–interaction because of the properties action–interacti of the magneticon properties forces of and the the magnetic significant forces temperature and the significantincrease due temperature to Joule eff increaseects. The due result to Joule will beeffects. a faster The failure result will of the be coila faster with failure increasing of the discharge coil with increasingenergies (Figure discharge 18a) forenergies a linear (Figure coil. As18a) previously for a linear mentioned, coil. As previously O-shape coils mentioned, provide higherO-shape impact coils velocitiesprovide higher compared impact to velocities straight coil, compared but what to aboutstraight the coil, life but cycle? what Table about 14 thepresents life cycle? the matrixTable 14 of presentstests with the di ffmatrixerent inductorsof tests with and different provides indu informationctors and on provides the cumulative information sustainable on the and cumulative average sustainableenergy/shot. and The average data analysis energy/shot. gives some The data trends analys withis dischargegives some energy trends and with coil discharge shape: energy and coil shape: • For a straight conductor, the life cycle is drastically reduced from 50 to 10 shots when the discharge energy is increased from 13 to 16 kJ; • For O-shape conductors, 50 shots with a discharge energy of 18 kJ are observed. This is the highest limit, but with decreasing discharge energy, a much higher number of weld shots could be seen, but this was not evaluated in real terms. An additional factor is the coil mounting system that has to maintain the inductor in place by counteracting the magnetic forces experimented by coil due to the action–reaction nature during repeated shocks. The magnetic forces occur not only between flyer and coil, but also within the internal parts of the coil itself. This is explicit in Table 14, which shows the effect of a strong coil hold (polyurethane, Figure 18b or by fiberglass/epoxy resin Figure 1c). As a general observation, polyurethane mount leads to coil distortion after a small number of shots (about five shots), while fiber glass/resin hold composite shows only joule heating (50 shots).

J. Manuf. Mater. Process. 2020, 4, 69 17 of 20

For a straight conductor, the life cycle is drastically reduced from 50 to 10 shots when the discharge • energy is increased from 13 to 16 kJ; For O-shape conductors, 50 shots with a discharge energy of 18 kJ are observed. This is the highest • limit, but with decreasing discharge energy, a much higher number of weld shots could be seen, but this was not evaluated in real terms.

J. Manuf.An Mater. additional Process. 2020 factor, 4, xis FOR the PEER coil REVIEW mounting system that has to maintain the inductor in place 18 of by21 counteracting the magnetic forces experimented by coil due to the action–reaction nature during repeatedTable shocks. 14. Matrix The of tests magnetic with different forces inductors occur not (in only blue betweenO-shape coil flyer with and polyurethane coil, but alsosupport, within in the internalyellow parts linear of coil the and coil in itself. a green This O-shape is explicit coil with in fiberglass/epoxy Table 14, which resin shows composite the eff support).ect of a strong coil hold (polyurethane, Figure 18b or by fiberglass/epoxyData resin Analysis Figure 1c). As a general observation, polyurethane mount leads to coil distortion after a small number of shots (about five shots), while fiber Inductor # Shots Per Energy Total Shots Total Cumulative Average Energy glass/resin hold composite shows only joule heating (50 shots). Energy (kJ) Usage

Table 14. Matrix5 6 of tests7 with8 di9 ﬀerent10 inductors11 12 13 (in blue14 O-shape15 16 coil17 with18 polyurethane support, in yellow linear coil and in a green O-shape coil with ﬁberglass/epoxy resin composite support). 9 0 0 0 0 0 0 0 0 0 0 0 3 0 0 3 48 16 Data Analysis 8 0 0 0 0 0 0 0 0 0 0 0 85 12 3 3 1 7 Total Average Inductor Total Shots Per Energy Cumulative Energy #7 0 0 0 0 0 3 0 0 3 0 0 3 0 Shots0 9 117 13 Energy (kJ) Usage 3 5 0 6 0 7 0 8 0 9 0 10 311 012 130 142 150 160 176 180 0 11 152 14 9 0 0 0 0 0 0 0 0 0 0 0 3 0 0 3 48 16 4 0 0 0 0 0 4 0 0 3 0 0 4 0 0 11 143 13 8 0 0 0 0 0 3 0 0 3 0 0 1 0 0 7 85 12 76 0 0 0 0 0 0 0 0 0 0 3 3 0 00 03 05 01 30 03 0 0 09 12 117 157 13 13 3 0 0 0 0 0 3 0 0 2 0 0 6 0 0 11 152 14 5 0 1 3 3 5 2 1 0 1 0 0 3 0 0 19 188 10 4 0 0 0 0 0 4 0 0 3 0 0 4 0 0 11 143 13 146 0 0 0 0 0 0 0 0 0 0 3 3 0 00 05 15 00 30 025 0 0 012 33 157 495 13 15 5 0 1 3 3 5 2 1 0 1 0 0 3 0 0 19 188 10 10 0 0 0 1 0 6 0 0 10 3 0 14 0 0 34 464 14 14 0 0 0 0 0 3 0 0 5 0 0 25 0 0 33 495 15 1013 0 0 0 0 0 0 1 0 0 0 6 7 0 00 108 34 07 140 016 0 0 134 43 464 590 14 14 13 0 0 0 0 0 7 0 8 4 7 0 16 0 1 43 590 14 12 0 0 0 0 0 15 0 0 16 0 0 16 0 0 47 614 13 12 0 0 0 0 0 15 0 0 16 0 0 16 0 0 47 614 13 2 5 5 1111 10 108 8 6 6 6 6 0 00 07 07 00 00 00 0 0 053 53 430 430 8 8

FigureFigure 18. EffectEffect of of different different levels of thermo-mechanical stresses for: ( (aa)) linear linear coil, coil, ( (bb)) O-shape O-shape coil coil with PU support, ( c)c) O-shape coil with fiberglass/epoxy fiberglass/epoxy resin compositecomposite support.support 4. Conclusions 4. Conclusions Electromagnetic pulse welding between similar (Al/Al) and dissimilar metals (Al/Steel) with Electromagnetic pulse welding between similar (Al/Al) and dissimilar metals (Al/Steel) with straight (I-shape) and O-shape coils has been investigated using two different layouts, i.e., straight straight (I-shape) and O-shape coils has been investigated using two different layouts, i.e., straight (MPW) or humped (MPSW) for the lap joints. The conclusions presented herein are relevant to (MPW) or humped (MPSW) for the lap joints. The conclusions presented herein are relevant to maximum discharge energies up to 16 kJ using copper and steel coils, and a flyer–parent standoff between 1 and 3 mm. The numerical simulation of straight and O-shape coils using coupled mechanical/thermal/electromagnetic using LS-DYNA 3D Finite Element Method–Boundary Element Method code (FEM-BEM) is employed as an asset to estimate the effect of coil shape on the welding of different alloyed Al–Steel couples. The conclusions of the numerical model are then validated by experimental production and characterization of lap welds in Al/Al, Al/steel couples of interests in automotive industry. The main conclusions can be summarized as follows:

J. Manuf. Mater. Process. 2020, 4, 69 18 of 20 maximum discharge energies up to 16 kJ using copper and steel coils, and a flyer–parent standoff between 1 and 3 mm. The numerical simulation of straight and O-shape coils using coupled mechanical/thermal/electromagnetic using LS-DYNA 3D Finite Element Method–Boundary Element Method code (FEM-BEM) is employed as an asset to estimate the effect of coil shape on the welding of different alloyed Al–Steel couples. The conclusions of the numerical model are then validated by experimental production and characterization of lap welds in Al/Al, Al/steel couples of interests in automotive industry. The main conclusions can be summarized as follows:

1. Simulation suggests that O-shape coils have higher process efficiency when measured on impact velocity criterion compared to straight (I-shape) coils for identical process parameters (discharge Energy, flyer–parent gap). Irrespective of the coil material, O-shape coils produce higher impact velocities which are propitious to effective welding. The maximum impact velocities of 550 and 900 m/s are, respectively, estimated for straight and O-shape copper coils. Impact velocity increases with standoff linearly for straight coils, contrary to O-shape coils where a velocity plateau seems to shift towards higher standoffs with increasing discharge energy. For instance, the plateau onset point moves from 1.6 to 2.1 mm when discharge energy goes up from 10 to 13 kJ for copper O-shape coil. At 16 kJ, the plateau onset is not observed for copper O-shape coil within the standoff of 2.8 mm; 2. Compared to steel, copper coil is estimated to generate superior impact velocities (900 m/s for copper O-shape coil vs. 710 m/s for steel O-shape coil at 16 kJ). Copper has a higher electrical conductivity—lower joule losses—but lower mechanical resistance than steel. Joule losses in steel coils end up with coil heating and would require cooling to maintain its stiffness in heavy duty applications as in automotive plants. The disadvantage with copper as coil material stem from the fact that coil gets heavy recoil pressure from the outgoing flyer sheet, which significantly reduces the life cycle of coil. Copper coils’ lifetime is reduced to a very few welds as the discharge energy increases; 3. The life cycle of the O-shape coil is significantly higher than that of the straight coil and is subject to how the coil is mounted in its holding system. Though reasonable from a theoretical point of view, this requires further study for a quantitative measure; 4. Higher impact velocities with O-shape coil, derived from numerical calculations, are indirectly validated by the possibility of welding additional couple sheets that were not straight coil at the tested discharge energies. For example, 1.4-mm thick 5182 was welded with 5754, DP450, and DC04; 2-mm thick 5182 with 5754, DP450, DC04, and, finally, 6013 with DP1000. As a general rule, thicker and materials with higher mechanical characteristics require superior discharge energies capable of providing higher impact velocities. When the generator has limited disposable energy, O-shape coil is a preferential choice for electromagnetic pulse welding; 5. Quasi-static, dynamic shear tests along with fatigue test results on different sheet combinations attest a higher resistance of the spot welds as fracture occurred outside the welds, mostly around the corners of the welded spots where applied stresses are concentrated.

The investigations reported here are focused on the application of electromagnetic pulses for the sheet metal welding in automotive applications in view of lightweight hybrid tailored structures, like body parts. The results do indicate a viable process approach and feasibility, as reported here.

Author Contributions: Conceptualization, C.K., S.M., G.R.; Methodology, C.K., G.R.; Software, C.K., G.R.; Validation, C.K., S.M., G.R.; Formal Analysis, C.K., S.M., G.R.; Investigation, C.K., G.R.; Resources, C.K., G.R.; Data Curation, C.K., S.M., G.R.; Writing—Original Draft Preparation, C.K., S.M., G.R.; Writing—Review and Editing, C.K., S.M., G.R.; Visualization, C.K., S.M., G.R.; Supervision, G.R.; Project Administration, G.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors wish to acknowledge Faurecia Group for the ﬁnancial support to conduct this study which was a part of an innovation program that took place from March 2014 to November 2017. J. Manuf. Mater. Process. 2020, 4, 69 19 of 20

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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